Since creation of the information theory by Shannon, for the purpose of increasing the speed and capacity of wireless communication systems, many studies have been made on transmission schemes to actualize the Shannon limit that is the upper limit of the transmissible communication rate with respect to a given radio communication channel. As techniques for asymptotically achieving the Shannon limit, there are coding and modulation, and in the coding scheme, turbo codes having the high error-correcting function are used frequently in the current wire less communication systems, and adopted in various standards.
Meanwhile, in the modulation scheme, optimal signal constellations in multilevel modulation have been studied to achieve larger transmission capacity. As the signal constellation, gray coding is the most popular, and as shown in FIG. 10, in symbols adjacent to one another in the perpendicular and horizontal direction, the symbols are placed so that only a single bit differs so as to minimize bit errors occurring in symbol error. Further, there exists a modulation scheme called multilevel coding. For example, when transmission of 4 bits is performed on a single transmission occasion, multilevel coding is a modulation scheme to constitute modulation signals as multilayer QPSK as shown in the figure.
Coding and modulation have been studied independently for a long time, and to aim at further asymptotic Shannon limit, studies have been started on coding and modulation schemes to combine both of the techniques for optimization. For example, Non-patent Document 1 proposes techniques for performing different coding for each layer in multilevel coding. In multilevel coding, as shown in FIG. 11B, the inter-signal-point distance of the layer 2 is different from the inter-signal-point distance of the layer 1. Therefore, when the same error-correcting code is used for each layer, transmission characteristics are different.
In other words, as compared with the layer 2, transmission characteristics of the layer 1 significantly deteriorate. Therefore, as shown in FIG. 12, Non-patent Document 1 proposes the communication system in which, when M layers exist, an information source is coded in coders with different coding rates for each layer, and a D-A converter (modulator) converts bits into symbols to transmit.
For example, when multilevel coding is performed as shown in FIGS. 11A and 11B, since the inter-signal-point distance is long in the layer 2, the coding rate is increased. Meanwhile, since the inter-signal-point distance is short in the layer 1, the coding rate is decreased. It is thereby possible to perform error-correcting coding with appropriate redundancy even in the case of providing amplitude with the information, and it is possible to enhance transmission characteristics of the entire transmission sequence.
Meanwhile, in recent years, studies have been also made on BICM-ID (Bit Interleaved Code Modulation with Iterative Detection) that achieves further improvements of transmission capacity by using iterative signal detection in the coding modulation scheme. For example, Non-patent Document 2 shows that it is possible to improve characteristics by inputting an output of a decoder to a demodulator as feedback to use as a priori information in demodulation.